find-unused-css: Fix shellcheck warnings.

In tools/find-unused-css line 5:
    if [ $(git grep "$n" | grep -v '^static/styles/zulip.css' | wc -l) -eq 0 ]; then
         ^-- SC2046: Quote this to prevent word splitting.
                           ^-- SC2126: Consider using grep -c instead of grep|wc -l.

In tools/find-unused-css line 6:
        echo $n
             ^-- SC2086: Double quote to prevent globbing and word splitting.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

tools/find-unused-css

@@ -2,7 +2,7 @@
 # Hackish tool for attempting to find unused IDs / classes in our CSS
 
 for n in $(perl -lne 'print $1 while /[#.]([a-zA-Z0-9_-]+)/g' static/styles/zulip.css | sort -u); do
-    if [ $(git grep "$n" | grep -v '^static/styles/zulip.css' | wc -l) -eq 0 ]; then
-        echo $n
+    if [ "$(git grep "$n" | grep -cv '^static/styles/zulip.css')" -eq 0 ]; then
+        echo "$n"
     fi
 done